DPF Maintenance

Every truck manufactured with a 2007 and later diesel engine is equipped with a diesel particulate filter. Many bus fleets started using them in 2005. There are something like 2 million DPFs, as they're called, in North America already.

Soon, on a phased-in program, every truck in California will also have to have a DPF, retrofitted to comply with California's mandate to reduce particulate emissions and their perceived health hazard. Other states are expected to follow California's lead.

At some point, these filters are going to require servicing, and that could well be an opportunity for independent service shops to pursue.

Some transit agencies have been using DPFs as retrofits since the mid-2000s, in duty cycles that are significantly less kind to the component than most on-highway truck applications. A linehaul truck may go 150,000 to 300,000 miles - sometimes more - before the filter needs maintenance. But transit and trash-truck managers may be looking to establish a filter maintenance program that cleans these expensive components every 50,000 miles.

About Diesel particulate filters

The DPF's job is to clean up particulate matter, or PM. The filters trap this microscopic carbon, cleaning up the exhaust to the point where a clean linen handkerchief placed over the exhaust remains completely clean and doesn't even smell of diesel exhaust.

The carbon trapped in the filter burns off in the excess air and nitrogen monoxide in the exhaust when the stream is hot enough. This depends on duty - a truck running up and down hills has enough heat to regenerate the filter passively.

But there are many trucks, and especially buses, running lighter duty cycles and there's not enough heat to light off the carbon particles. In the early days of passive-only filters, this meant removing the filter element when the backpressure light came on and cleaning out the accumulated particulate matter and any ash that came over from lube oils. (A new oil standard, CJ-4, requires lower levels of ash-causing components.) But with the DPF appearing on trucks in 2007, regeneration on the vehicle has been available, with the "active" regeneration promoted by injected diesel fuel into the exhaust stream heating up the filter. But the ash from the oil remains, and it has to be removed on a regular basis - maybe as often as every 150,000 miles.

This is the point at which the filter must be serviced.

Handle with care

When it comes to handling a component that can cost as much as $8,000 if damaged, it pays to have a program that treats the filter element kindly. So it was standing room only at a workshop on diesel particulate filter maintenance at the fall meeting of the Technology and Maintenance Council of the American Trucking Associations. The workshop was co-presented by Chuck Blake of Detroit Diesel, Kevin Otto of Cummins, Brent Cluskey of Caterpillar and Vince Lindley of Volvo.

The last thing you want is the technician removing the filter from the exhaust system and banging it on the shop floor to clear out accumulated ash. That would be bad for two reasons. One, the ash is not classified as hazardous in most places, but you still wouldn't want your shop staff exposed to its fine particles. Two, bang the delicate filter on the floor and you'll break it.

Correct handling and cleaning of the filter is something fleets may well want to avoid. So an outside service that has experience and the equipment to correctly and thoroughly clean filters represents an opportunity for the aftermarket. And based on the business plan you can download from cleaner manufacturer FSX, it can be very lucrative indeed. In the company's analysis, clean 30 filters a week and your contribution to the year's bottom line will be over $325,000 net cash flow.

Problems with filters

Besides the build-up of ash, there can be other problems with diesel particulate filters. The presenters at the TMC session mentioned some that are mostly a result of mechanical failure.

This could be of the filter element or, as it is more correctly described, the ceramic monolith. It is one piece of ceramic - hence monolith - that must retain its integrity in order to cause the diesel exhaust to pass only through the fine-filtering walls of the ceramic material. If it is cracked either through poor handling by the technician, through vibration, or because the driver has driven the truck across railway lines and bashed the DPF, it will allow diesel particulates to the downstream side of the filter. Thus, when tearing down to service the filter, if any black deposits are found on the downstream side of the filter, the monolith is bad and must be replaced.

The other early removal cause is through an engine problem, usually a turbo failure that allows lube oil into the filter matrix, or a leaky injector that loads it with raw fuel. Both block the through-the-wall flow of exhaust and produce a sudden rise in backpressure.

Getting the ash out

Ash is the long-term and predictable reason for filter service in over-the-road trucks.

With fleets running more and more post-2007 trucks, there has been a gradual move to the latest, low-ash formulation CJ-4 oil that is recommended for use with DPFs. But CJ-4 oil is usually more expensive than earlier formulations, so it has been slower to catch on. When older oil formulations are used, ash may build up sooner in the DPF. The result is potentially more filter servicing work for the right shop.

Another issue is the amount of sulfur in the fuel. Since late 2006, most on-highway fueling locations provide ultra-low sulfur fuel with 15 ppm rather than 500 ppm sulfur content to protect the DPF and the precious-metal catalysts on the surface of the ceramic. It is believed that one or two fuel loads of the higher sulfur fuel will not do any serious harm to the filter, but more could cause a reduced service life for the element.

For over-the-road trucks, DPF service is predicted to be anything from 150,000 to 300,000 miles at the earliest. In cleaning, the filter element is returned to a percentage of its as-new capacity that can vary from around two-thirds to almost as good as new. But there's considerable debate about which process is better.

Servicing the unit

Most DPF systems are designed to be quick and easy to service. But there are different service requirements. Volvo, Mack and Cummins all use the FleetGuard systems that require an air blast cleaning in a purpose-made cabinet. Detroit Diesel says its DPF is best cleaned with de-ionized water, although more recent service bulletins have announced air blast cleaning is acceptable. Early on, Caterpillar had a vacuum-cleaner like device that doesn't require disassembling the filter system, though more recently Cat dealers offer air-pulse cleaning off the truck using a Caterpillar/SPX developed cleaning machine. Paccar recommends Kenworth and Peterbilt dealers use a system by FSX that individually blasts air down each cell of the filter from both ends, rather than just pulsing the whole assembly from one side.

In most cases the filters are designed to be easy to access. They require simple hand tools and a jack rather like a transmission jack for the low-mounted tea-kettle type DPFs found on Volvo and Mack trucks. Peterbilts and Kenworths have low-mounted cylindrical DPFs, but there are also stack-mounted units. All are heavy enough to require some support as they are serviced.

In most designs, the filter housing is held together with easily removed bands so the filter assembly can be separated and a new filter installed in 30 minutes to an hour. The actual cleaning process on the machine takes around 20 to 45 minutes, and the general recommendation is that a cleaning can be accommodated during an oil change.

A special case that calls for filter cleansing is contamination by engine lubricant or unburned fuel such as may oc